|
I
have bought a generator, and I am concerned about safety. Can
I use a personal power breaker?
Answer
: The generator is configured differently to the mains supply.
The generator has a ‘floating earth’, whilst the mains
has an earthed neutral. Whereas it is definitely recommended to
use a personal power breaker (RCD) from the mains, for the majority
of cases, it is not necessary to use one with a B&S generator.
The generators are safe as they are designed.
Personal power breakers are designed to operate from the mains.
If one is to be used with a generator, then it is necessary to
modify the generator so that it is configured in the same way
as the mains. This is a relatively simple modification for a qualified
electrician involving adding a link wire from the neutral terminal
to the earth terminal. However, once the generator has been modified,
it is necessary to then always use a personal power breaker and
to also always use an earth spike, which connects between the
generator frame and the ground. Since this is difficult to ensure,
it is generally recommended not to modify the generator.
Question
: What is the difference between kW and kVA? What is Power factor?
Answer
: There are 3 types of electrical:
Real
Power, measured in Watts (W). This is the power drawn by a resistive
load, e.g. a heater element in a kettle, and has a power factor
of 1. (unity power factor, cos F=1, 1.0pf or pf=1)
Reactive
Power, measured in Volt Amperes reactive (VAr’s). This is
the power drawn by a reactive load (a load with a winding around
a core), e.g. an electro-magnet, and has a power factor of 0.
(zero power factor, cos F =0, 0pf or pf=0)
Apparent
Power, measured in Volt Amperes (VA). Many loads have a combination
of resistive and reactive elements. (in fact it is not possible
to produce a purely inductive load, since the wire used to form
the windings has a resistance). This combination of elements means
that both real power (W) and reactive power (VAr) are drawn together.
The
proportion of Real Power to Reactive Power is defined as the power
factor. [Nearly all resistive load (e.g. Universal motor used
in hand tools) then power factor 0.95 to 1.0, nearly all inductive
load then power factor ~ 0.3]
The vast majority of single-phase loads have power factors approaching
1. Therefore, single-phase generator power ratings are taken at
power factor =1, and are consequently in Watts (W) or kilo Watts
(kW), where 1 kW = 1000 W.
Three-phase
loads tend to have lower power factors, approaching 0.8, therefore,
three phase generator power ratings are taken at power factor
=0.8 and are in VA or kVA.
There
is obviously a relationship between real power, reactive power,
apparent power and power factor….
i)
Apparent Power (VA) = Ö [(real power (W))2 + (reactive power
(VAr))2]
And
ii)
Power factor = Real Power (W)
Apparent Power (VA)
Therefore…
Apparent
Power (VA) x Power factor = Real power (W)
If the Power factor =1, then all the Power is real, and
Apparent Power (VA) = Real Power (W)
(W = VA @ 1.0 pf)
For
a single-phase generator, the rating should be at 1.0 pf, in which
case
Watts = Volt Amperes.
But, for a three-phase generator the rating is at 0.8pf.
This
is where confusion can arise!
Example.
A three-phase generator has a continuous rating of 5 kVA at 0.8
pf.
Now,
at this rated load, the Real power (kW) will be …
Real
Power (kW) = Apparent Power (kVA) x Power factor
Real
Power = 5 x 0.8 = 4kW
This
means that a generator producing 5kVA at 0.8pf is actually producing
4 kW of Real power, but it is also producing some reactive power.
From
i)…
5000 VA = Ö [( 4000 W)2 + ( Reactive Power)2]
Reactive Power = 3000 VAr’s
It
is this combination of 4kW of real power and 3kVAr’s of reactive
power that has defined the limit for the generator rating.
If
the same generator was loaded with a resistive load only, then
it may be capable of more than 4kW, however, there is no formula
that can be used to find this limit from the 0.8pf rating. It
can only be found through testing of each machine.
Similarly, a single-phase generator rated at 4kW, cannot be expected
to produce 5kVA at 0.8pf !!!!
Question
: Can I operate sensitive equipment from a generator?
Answer
: The output from a portable generator is not as stable as the
supply from the mains. The speed of the engine driving the alternator
is controlled by a simple mechanical governor; consequently, the
speed drops as the load is increased. The frequency of the output
voltage is directly dependent on the engine speed; therefore,
the frequency of the output varies with load. In addition, the
output voltage will vary with load, and with temperature. The
output voltage of all Generac generators will remain within 230V
+/- 10% from no load up to the rated load current quoted on the
dataplate. This is the guaranteed range of voltage supplied from
the mains utilities.
The
frequency of the output voltage will vary typically from 53Hz
at no load to 49Hz at rated load current, whereas the mains supply
is unlikely to vary by more than 0.1 Hz.
Most electronic equipment is designed to cope with these fluctuations
and will operate normally. However, it is always recommended to
ask the equipment supplier whether their equipment is suitable
to be operated from a portable generator.
As a generator runs out of fuel, the engine is likely to surge.
To avoid this affecting electronic equipment an Uninterruptible
Power Supply (UPS) can be used. These are typically sold for use
with computers so that data is not lost in the event of a power
cut.
What
size generator do I need to start a motor ?
Answer
: This is a tricky one, since there are many different designs
of motor, each with different characteristics
Some motors, e.g. Induction type motors (capacitor start/capacitor
run) require additional current to start them, therefore requiring
a larger generator.
Motors fitted to hand tools generally do not require any additional
start-up current.
Consequently, it is recommended to ask the supplier of the equipment
that you wish to run whether it requires additional start-up current.
As a rough guide only, allow for a generator that has a continuous
rating of 2 ½ to 3 times the motor rating.
Motors can be rated in kW or HP. To convert HP to kW, multiply
by ¾.
E.g. What generator to run a 3HP motor?
3HP = 3 x ¾ = 2.25kW.
This motor would require a generator of between;
(2 ½ x 2.25) = 5.625kW
and …
(3 x 2.25) = 6.75kW
Question
: I have bought a generator, and would like to connect it to my
house in case of mains failure. What do I need to do?
Answer
: When using a generator as an alternative supply to the mains,
there are several precautions that must be observed.
It is vital that the generator is completely isolated from the
mains supply. This ensures that the generator is not attempting
to power up the whole neighborhood, but also ensures that it does
not electrocute a utility worker trying to restore the mains supply.
To
achieve this, a double-pole, break-before-make, changeover switch
must be installed by a qualified electrician. Briggs & Stratton
manufacture a manual transfer switch for this application. Click
Here To View the BTS9200M.
This should be fitted between the electricity meter and the building
consumer unit. The switch connects the building to either the
mains supply or to a lead which can be plugged into the generator.
Most
buildings now have an RCD built into the consumer unit. This is
configured to operate from the mains supply with an earthed neutral,
and not from a generator with a floating earth. To utilize this
protection device, it is necessary to modify the generator so
that it is configured in the same way as the mains supply. This
is a simple modification for a qualified electrician, involving
adding a link wire from the neutral terminal to the earth terminal.
It is recommended to make this connection in the plug that is
to be used to connect to the generator. This ensures that the
generator is unmodified when it is disconnected from the house,
and therefore remains safe.
The
plug should be labeled “Do not connect to mains: Neutral-Earth
link fitted”. The lead between the generator and the transfer
switch is not protected by the RCD, it is therefore recommended
to use a steel armored cable for this connection.
Finally a local low-impedance earth spike needs to be installed
What
size generator do I need for my welder ?
Answer
: A welder is rated by its output current. To estimate its
input power, divide the output rating by 30.
E.g. A 130A welder will have an input requirement of approximately
130/30=4.3kW. A 200A welder will have an input requirement of
approximately 200/30=6.7kW.
This is only an estimate; therefore, it is recommended to choose
a generator of the next size up. However, bear in mind, that many
users will not actually require the full capacity of their welder,
a smaller generator would still operate the welder, but would
limit the welding current.
|